Within the past 40 years, tremendous progress has been made in both the efficiency and cost reduction of photovoltaic (PV) cells that convert sunlight to electricity. However, one of the main limitations of using solar power as an energy source is that the electricity must be used immediately or stored in a secondary device . Photoelectrochemical (PEC) cells combined in tandem with PV cells offer a solution to this problem by using solar radiation (light) to electrolyze water and generate hydrogen which can then be converted to electricity using fuel cells or be used to synthesize and store hydrocarbon fuels by hydrogenation of CO2 .
Iron oxide (also known as α-Fe2O3, hematite, or rust) has become the leading candidate for use as a photoanode material in PEC cells due to its stability in aqueous solutions, 2.1 eV band gap, vast abundance, and low cost. However, hematite has extremely poor charge carrier mobility, and a very short lifetime of photogenerated carriers. The combination of these two characteristics results in a collection length of only 2-20 nm for the photogenerated minority charge carriers (holes). This is significantly shorter than the optical absorption depth of light in hematite. As a result, significant bulk recombination occurs for thicker films on planar substrates, preventing the holes from reaching the semiconductor/liquid interface and thereby reducing efficiency. The host’s (Prof. Avner Rothschild) research group at the Technion Institute of Technology invented a new approach towards overcoming the trade-off between light absorption and charge carrier collection length. Rather than decoupling the optical and electrical path lengths, a significant amount of light is instead trapped in ultrathin films. Nevertheless, state-of-the-art hematite photoanodes still operate at approximately 30-40% of the theoretical efficiency.
This project focused on a multi-faceted research plan that addressed the salient challenges facing efficient water splitting in hematite photoanodes, seeking to improve the photoconversion efficiency of ultrathin film α-Fe2O3 photoanodes and gain fundamental understanding of the effects of thin film processing on the structure and properties of α-Fe2O3 ultrathin films. Four specific objectives were listed and met by a combination of thin film engineering coupled with materials, electrical, and photoelectrochemical characterization. These objectives included the development of heteroepitaxial hematite thin film photoanodes, determination of optimal doping profiles, development of selective hole and electron blocking layers, and investigation of orientation dependent properties of hematite photoanodes.
